Joining product of oxide superconducting material and process for producing the same

ABSTRACT

A joining product of oxide superconducting materials having a high current density and process for producing the same. A joining product comprising a plurality of oxide superconducting materials having an identical crystal orientation joined with each other through a superconducting phase of the same type as described above which has the same crystal orientation as the oxide superconducting materials and a lower peritectic temperature than the oxide superconducting materials. A joining method comprising the steps of: regulating oxide superconducting materials to be joined so that they have an identical crystal orientation; either inserting as a solder a material comprising elements constituting an oxide superconductor having a lower peritectic temperature than the oxide superconducting materials or bringing the material comprising elements constituting an oxide superconductor having a lower peritectic temperature than the oxide superconducting materials into contact with the oxide superconducting materials to be joined; heating the assembly to a temperature below the peritectic temperature of the superconducting materials to be joined and above the peritectic temperature of the solder; and gradually cooling the heated assembly to orient and grow the same type of oxide superconductor at the joining interface. A junction product of oxide superconducting materials free from such crystal grain boundary as to inhibit the current flow and having a high critical current density can be provided and can be utilized as a magnet, a magnetic shield and a current leading material.

TECHNICAL FIELD

The present invention relates to a REBa₂ Cu₃ O_(x) -basedsuperconducting joined product having a high critical current densityand a process for producing the same.

BACKGROUND ART

REBa₂ Cu₃ O_(x) -based superconductors have a high critical temperatureexceeding the liquid nitrogen temperature and hence can offer a greateconomical benefit when they could be put to practical use. In thesesuperconductors, however, a crystal grain boundary serves as a weaklink, so that a high superconducting current cannot be flowed across thecrystal grain boundary and, in particular, the critical current densityis remarkably lowered in a magnetic field. For this reason, thepractical use of the REBa₂ Cu₃ O_(x) -based superconductors could havenot been realized in the field of heavy current.

The production of a REBa₂ Cu₃ O_(x) -based superconducting bulk materialcomprising large crystal grains having a volume of not less than 50 cm³has become possible (see M. Morita. et al: Advances in superconductivityIII, Springer-Verlag, Tokyo, 1990, pp. 733-736) by the melt process,which is a process for producing an oxide superconducting material,including the QMG process (quench and melt growth process) (see U.S.patent application Ser. No. 07/735,105, now U.S. Pat. No. 5,278,137, orJapanese Unexamined Patent Publication (Kokai) No. 2-153,803). The meltprocess basically comprises heating a starting material for a REBa₂ Cu₃O_(x) -based superconductor to a semi-melted state wherein a RE₂ BaCuO₅phase is present together with a liquid phase composed mainly of Ba, Cuand O; and gradually cooling the system from just above the peritectictemperature to conduct crystal growth of the REBa₂ Cu₃ O_(x) phase.Large grains prepared by this process contains a small angle grainboundary having a crystal misorientation of several degrees but no grainboundary having a large angle which serves as a weak link. For thisreason, in this material, the critical current density within the grainsis as high as 10000 A/cm² or more at 77 K and 1 T, so that the materialis considered to be applicable as materials for superconducting coils,bulk magnets, magnetic shields and current leads.

In order that the REBa₂ Cu₃ O_(x) -based superconducting bulk materialprepared by the melt process can be used as these materials, it isnecessary to increase the size, area or length. Since, however, the meltprocess is basically a technique for producing an oxide superconductorhaving no grain boundary by gradual cooling from the melted state, thereis a limitation on an increase in size. Further, the material producedby the melt process cannot be subjected to working which breaks crystalgrains, such as rolling and bending. Therefore, it is considered that ifit becomes possible to prepare a junction having a high critical currentdensity, an increase in size of the material and the shaping becomeeasy, which enables the material to be used in the above applications.

Joining methods for an oxide superconductor, which have been reported inthe art, include a method which utilizes solid phase diffusion and amethod wherein a liquid phase is formed to conduct joining. Most ofthese joining methods are intended for use in joining between sinteredbodies, and no consideration is taken for the prevention of theformation of a grain boundary in the junction. Therefore, the formationof a grain boundary in the junction is unavoidable. In this case, thecrystal grain boundary serves as a weak link, and no high criticalcurrent density can be obtained. Further, also when the material is usedas a magnetic shielding material, no good shielding property can beobtained because the grain boundary permits a magnetic field topenetrate. No joining process has hitherto been reported which isintended for use in a material free from a grain boundary in thejunction and having a high critical current density. Specific examplesof the convention process will now be described.

Examples of methods for conducting joining between oriented REBa₂ Cu₃O_(x) -based superconductors include a method which comprises insertinga single crystal powder of the same superconductor as those to be joinedbetween junction bulks and conducting sintering under pressure in amagnetic field (see Japanese Unexamined Patent Publication (Kokai) No.2-270279) and a method which comprises polishing the joining interfaceand then conducting sintering in a solid phase while applying pressure(see K. Salama et al., Appl. Phys. Lett. 60 (1992), p.898). However,according to reproduction tests on these joining processes conducted bythe present inventors, the above joining processes in a solid phaseprovide no junction having a satisfactory high strength and a highcritical current density.

A process which comprises once forming a liquid phase in the joininginterface and then conducting joining is considered advantageous fromthe viewpoint of strength. Methods which comprise forming a liquid phaseand bringing also the joining interface to a superconducting stateinclude a method which comprises locally heating the junction by meansof a laser beam to a temperature above the peritectic temperature andthen conducting solidification for joining (see Japanese UnexaminedPatent Publication (Kokai) No. 2-82482) and a method wherein use is madeof a brazing material which has a low melting point (peritectictemperature) and can be brought to a superconducting state uponsolidification (see Japanese Unexamined Patent Publication (Kokai) Nos.63-291377, 1-242473 and 3-237073 and U.S. Pat. Nos. 5116810 and5079226). In these methods, however, melting and solidification aremerely carried out, and no means for preventing the formation of a grainboundary in the joining interface is used. In these prior art, workingexamples are also directed to joining of sintered bodies.

In sintered body, the crystal orientation arbitrarily varies from grainto grain, so that it cannot be said that the crystal orientation isuniform. Therefore, in joined products produced by the above methods,the crystal grain boundary serves as a weak link, so that no highcritical current density can be obtained. In particular, in a magneticfield, since a lowering in critical current density is so significantthat the joined-product having a weak link is unsuitable as a materialfor constituting a magnet generating a strong magnetic field, ashielding material for shielding a magnetic field having a mediumstrength or a current leading material for flowing a large current.

An object of the present invention is to provide a joined productcomprising REBa₂ Cu₃ O_(x) -based superconducting materials and having ajunction free from any large angle grain boundary and a method forjoining the REBa₂ Cu₃ O_(x) -based superconducting materials.

DISCLOSURE OF INVENTION

In order to solve the above-described problem, the present inventionprovides a joining process which comprises aligning highly orientedREBa₂ Cu₃ O_(x) -based superconducting materials to be joined so as tohave an identical crystal orientation; either inserting as a solder aREBa₂ Cu₃ O_(x) -based superconducting material of a composition havinga lower peritectic temperature than the materials to be joined or a rawmaterial therefor between the materials to be joined or bringing thesolder (junction material) into contact with the materials to be joinedand applying the solder to the materials to be joined so that it sitsastride on the materials to be joined; heating the assembly to such anextent that the REBa₂ Cu₃ O_(x) -based superconducting materials to bejoined remain stable with the solder being in a semi-melted statecomprising a RE₂ BaCuO₅ phase and, present together therewith, a liquidphase composed mainly of Ba, Cu and O; and gradually cooling the heatedassembly to cause a peritectic reaction in the solder portion, therebyforming and growing REBa₂ Cu₃ O, while maintaining the crystalorientation of the REBa₂ Cu₃ O_(x) -based superconducting materials tobe joined, whereby a junction having a high critical current density isprepared without occurrence of any large angle grain boundary.

In the joining process according to the present invention, the followingrequirements should be met.

1 REBa₂ Cu₃ O_(x) -based superconducting materials to be joined shouldhave an orientation.

2 Oriented REBa₂ Cu₃ O_(x) -based superconducting materials to be joinedshould be aligned so that they have an identical crystal orientation,and a solder should be placed and adhered to the materials on theirsurfaces to be joined.

3 The solder should be a material which has a lower peritectictemperature than the REBa₂ Cu₃ O_(x) -based superconducting materials tobe joined and, upon melting and solidification, can be brought to asuperconductor having the same crystal structure as the REBa₂ Cu₃ O_(x)-based superconducting materials to be joined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the first embodiment of joining;

FIG. 2 is a schematic diagram showing the second embodiment of joining;

FIG. 3 is a schematic diagram showing the third embodiment of joining;

FIG. 4 is a pseudo-binary diagram of REBa₂ Cu₃ O_(x) ;

FIG. 5 is a perspective view showing a positional relationship injoining in a working example;

FIG. 6 is a perspective view showing a positional relationship betweensamples to be joined and a joining solder in a working example;

FIG. 7 is a perspective view showing a positional relationship of asilver electrode as a film formed for the measurement of a criticalcurrent density in a working example;

FIG. 8 is an optical microphotograph of a junction;

FIG. 9 is an absorption electron image in the vicinity of a joininginterface;

FIG. 10 is a graph showing a concentration distribution of Yb crossing ajoining layer as measured by EPMA;

FIG. 11 is a perspective view showing a positional relationship betweena sample to be joined and a joining solder in a working example;

FIG. 12 is a critical current of a junction measured in a magnetic fieldin a working example;

FIGS. 13 and 14 are optical (polarizing) microphotographs of junctions;

FIG. 15 is a diffraction peak of a joining area from a (006) plane byX-ray diffractometry;

FIGS. 16 and 17 are perspective views showing a positional relationshipbetween samples to be joined and a joining solder in working examples;

FIG. 18 is a perspective view showing a positional relationship betweensamples to be joined and a joining solder in a working example;

FIG. 19 is a perspective view showing the position of a Hall probe of asample (sample C), which has been gradually cooled at a rate of 2°C./hr, and a current flow after cooling in a magnetic field, and

FIG. 20 is a perspective view showing the position of a Hall probe of asample (sample D), which has been cooled at a rate of 300° C./hr, and acurrent flow after cooling in a magnetic field;

FIG. 21 is a graph showing the trapped magnetic flux densitydistribution measured after cooling in a magnetic field in a workingexample;

FIG. 22 is a perspective view showing an embodiment wherein a joinedproduct has been used in a current leading conductor;

FIGS. 23 and 24 are perspective views showing embodiments wherein joinedproducts have been used respectively in a magnetic shielding materialand a bulk magnet; and

FIG. 25 is a perspective view showing an embodiment wherein a joinedproduct has been used in a superconducting coil.

BEST MODE FOR CARRYING OUT THE INVENTION

Materials prepared by the above-described QMG process or otherprocesses, oriented films, etc., are considered usable as REBa₂ Cu₃O_(x) -based superconductors to be joined. As described above, thematerials having an identical crystal orientation used herein refers tolarge crystals which may have a small angle grain boundary having acrystal misorientation of several degrees but are free from a grainboundary having a large angle grain boundary serving as a weak link. TheREBa₂ Cu₃ O_(x) -based superconductors to be joined need not have anidentical composition so far as they have superconducting properties.

The highly oriented REBa₂ Cu₃ O_(x) -based superconductors may bejoined, for example, according to an embodiment as shown in FIGS. 1 and2 wherein a solder is inserted as a joining layer betweensuperconductors 1 to be joined and joining is then carried out and anembodiment as shown in FIG. 3 wherein a joining bulk (a solder or ajunction material) 2 is joined to each of the superconductors 1 withoutinserting any solder between the superconductors 1 to be joined. In theembodiment as shown in FIG. 3, the joining phase 2 per se may occupy ahigh proportion of the final superconducting joined product. The face tobe joined may not be always flat and may be modified, for example,rendered uneven.

In particular, in the embodiment shown in FIG. 3, a bulk may be used asthe solder 2. In this case, as is apparent from the phase diagram shownin FIG. 4, which will be described joining bulk, when the bulk isheat-treated, it is decomposed into a RE₂ BaCuO₅ phase and a liquidphase composed mainly of Ba, Cu and O. Since the RE₂ BaCuO₅ phase issolid, it is transformed to REBa₂ Cu₃ O_(x) while maintaining the shapeof the bulk. In the present invention, the term "solder" is used inmeaning including such a joining bulk.

Further, the REBa₂ Cu₃ O_(x) -based superconductor to be joined may bein the form of a superconductor film formed on a substrate as well as inthe form of a bulk superconductor.

The following method is considered for the preparation of a solder whichhas a lower peritectic temperature than the REBa₂ Cu₃ O_(x) -basedmaterials to be joined and, upon cooling, becomes a REBa₂ Cu₃ O_(x)-based superconducting material of the same type as the REBa₂ Cu₃ O_(x)-based materials to be joined.

At the outset, substitution of RE is carried out. FIG. 4 shows aschematic pseudo-binary phase diagram of a REBa₂ Cu₃ O_(x) -basedmaterial. In this case, RE is at least one element selected from thegroup consisting of Y, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu. Thematerial having a composition close to REBa₂ Cu₃ O_(x) is decomposed ata high temperature into a RE₂ BaCuO₅ phase (211) and a liquid phase (L)composed mainly of Ba, Cu and O. When this liquid phase is cooled, itforms REBa₂ Cu₃ O_(x) through a peritectic reaction. The peritectictemperature (T) varies depending upon the kind of RE.

The peritectic temperatures (T) of representative REBa₂ Cu₃ O_(x)wherein RE comprises one element are summarized bellow.

    ______________________________________                                        RE      Peritectic    RE      Peritectic                                      element temp. (°C.)                                                                          element temp. (°C.)                              ______________________________________                                        Y       1000          Ho      990                                             Sm      1060          Er      970                                             Eu      1050          Tm      940                                             Gd      1030          Yb      900                                             Dy      1010                                                                  ______________________________________                                    

These are values determined in the air, and the peritectic temperaturevaries depending upon the partial pressure of oxygen. For example, theperitectic temperature increases by about 30° C. in pure oxygen of 1atm. In general, the smaller the ion radius, the lower the peritectictemperature. When RE comprises two elements, the peritectic temperatureis intermediate between the peritectic temperatures of these twoelements. Nd does not form RE₂ BaCuO₅ even at a high temperature, andsubstitution of a part of RE in REBa₂ Cu₃ O_(x) with Nd can increase theperitectic temperature.

This phenomenon may be utilized as follows. A system having a lowerperitectic temperature than the REBa₂ Cu₃ O_(x) materials to be joined,wherein RE is substituted, is selected as a solder to form a liquidphase in the junction at a temperature below the peritectic temperatureof REBa₂ Cu₃ O_(x) to be joined, which liquid phase is then cooled toform REBa₂ Cu₃ O_(x) in the junction, thereby forming a junction throughwhich a superconducting current can be flowed. Before the heattreatment, the solder need not be in the form of a REBa₂ Cu₃ O_(x)-based material and may be in the form of a raw material for REBa₂ Cu₃O_(x), such as oxides or double oxides of elements constituting thecomposition. Further, the composition may not be always stoichiometricand may have a ratio of RE:Ba:Cu deviated from 1:2:3 and contain minoramounts of impurities so far as the superconducting properties are notspoiled. The ratio of RE to Ba to Cu (RE:Ba:Cu) is preferably in aregion formed by connecting points (10:60:30), (10:20:70) and (50:20:30)in terms of atomic %.

The second method is to add Ag or Au element, which serves to lower theperitectic temperature of REBa₂ Cu₃ O_(x), to the solder. In thismethod, RE in the materials to be joined may be the same as RE in thesolder. Since, however, the peritectic temperature difference is notlarger than that in the first method, this method is preferably used incombination with the substitution of RE element. The Ag or Au elementmay be introduced, for example, in the form of an oxide such as Ag₂ O.

Examples of the method for the disposition of the solder include amethod wherein the joining area is merely covered with a solder powder,a method wherein a molded, sintered or melting-solidified product of asolder powder is inserted and disposed, a method wherein a mixture of asolder powder with an organic binder is coated and a method wherein afilm is formed on the joining area by the sol-gel process or using afilm-forming device, such as a sputtering device.

When the REBa₂ Cu₃ O_(x) -based superconducting materials to be joineddo not have an identical crystal orientation, there occurs a differencein crystal lattice, that is, a grain boundary, between joining phases.This grain boundary serves as a weak link to unfavorably lower thecritical current density. For this reason, the REBa₂ Cu₃ O_(x) -basedsuperconducting materials to be joined should be aligned and free from aweak link and further have an identical orientation. It is consideredthat the difference in crystal orientation can be relaxed to some extentby the joining phase. However, it is particularly desired that thecrystal misorientation in the direction of the c-axis and the a-/b-axisbe not more than 150. The "c-axis" used herein is defined as the longestaxis in a unit lattice of REBa₂ Cu₃ O_(x) having an orthorhombicstructure, and the "a- and b-axes" are defined as two axes other thanthe c axis. When the misorientation in these axes exceeds 15°, a joinedproduct having a high critical current density, which is a feature ofthe present invention, cannot be provided, as will be apparent fromexamples which will be described later.

Although the REBa₂ Cu₃ O_(x) superconductor is orthorhombic, itmicroscopically has a twin structure wherein the a- and b-axes arepresent together in such a state that they are mutually deviated by 900,the a- and b-axes being macroscopically indistinguishable from eachother. The boundary of the twin where the crystal axes are deviated by900 does not serve as a weak link. However, when the crystal axes aredeviated by an intermediate angle from these axes (therefore 45° C. atthe maximum), even though the REBa₂ Cu₃ O_(x) superconductors have anidentical crystal orientation in the direction of the c-axis, thisportion unfavorably serves as a weak bond. That the crystalmisorientation in the direction of the a-/b-axis is desirably not morethan 15° means that although the REBa₂ Cu₃ O_(x) may take a twinstructure wherein the a- and b-axes are present together in such a statethat they are mutually deviated by 90°, the deviation from these axes isdesirably not more than 15°. The a-axis and b-axis, which aremacroscopically indistinguishable from each other, will be referred toas "a-/b-axis."

The composition of the REBa₂ Cu₃ O_(x) -based superconducting materialsto be joined may not be always stoichiometric and may have a ratio ofRE:Ba:Cu deviated from 1:2:3 and contain minor amounts of impurities sofar as the superconducting properties are not spoiled. The ratio of REto Ba to Cu (RE:Ba:Cu) is preferably in a region formed by connectingpoints (10:60:30), (10:20:70) and (50:20:30) in terms of atomic %.

In the joined product which has been heated at a temperature below theperitectic temperature of the REBa₂ Cu₃ O_(x) -based superconductingmaterials to be joined and above the peritectic temperature of thejoining solder, the joining solder portion is decomposed into a RE₂BaCuO₅ phase and a liquid phase composed mainly of Ba, Cu and O, and theRE elements are slightly diffused in each other at the interface of theREBa₂ Cu₃ O_(x) -based superconductors which are being joined to theliquid phase. In order to increase the junction strength, the heatingtemperature is preferably as high as possible so far as a fracture ofthe crystal structure of the REBa₂ Cu₃ O_(x) -based materials to bejoined, which results in a deterioration in properties, does not occur.Further, in order to avoid the deterioration in properties, the junctionmay be locally heated by using laser as an auxiliary means.

In order to increase the adhesion between the REBa₂ Cu₃ O_(x) -basedsuperconducting materials to be joined and the solder, it is desirableto apply pressure from the direction perpendicular to the junction area.

When cooling is then carried out, REBa₂ Cu₃ O_(x) is formed around theperitectic temperature of the solder. In this case, when cooling iscarried out gradually around this temperature, in the solder portion,the REBa₂ Cu₃ O_(x) crystal nucleates and grows inheriting theorientation of the interface of the REBa₂ Cu₃ O_(x) to be joined. Thecrystal of REBa₂ Cu₃ O_(x), which have been newly produced from bothsides of the joining area, grow and are finally joined at theintermediate portion. In this case, since the newly formed REBa₂ Cu₃O_(x) crystals have an identical crystal orientation, no crystal grainboundary occurs in the joining area, so that no weak link is formed.When gradual cooling is not carried out around this temperature, thesupercooling becomes so high that the nucleation of REBa₂ Cu₃ O_(x)occurs at the intermediate portion, or the growth rate does not followthereup. As a result, polycrystallization occurs in the junction, orcrystals are not completely connected to one another. The reason why thegradual cooling is carried out around the peritectic temperature of thesolder is that it is considered that mutual diffusion occurs at theinterface of the REBa₂ Cu₃ O_(x) materials to be joined with the solder,so that the peritectic temperature at the interface is higher than thatof the solder per se.

Therefore, the average cooling rate is preferably not more than 100/d(°C./hr), still preferably not more than 40/d (°C./hr), at least in atemperature range of (the peritectic temperature of the solder) ±5° C.based on experience although it depends also upon the thickness of thesolder and the temperature gradient of the furnace. In this case, d isthe thickness (mm) of the joining layer. The reason why the cooling rateis expressed in average value is that cooling may not be always carriedout at a constant rate, and the same effect can be attained also whenthe materials are held at a constant temperature just below theperitectic temperature. Gradual cooling is preferred also from theviewpoint of avoiding the formation of cracks due to thermal stress.

Thus, according to the present invention, a junction free from a crystalboundary in the junction and having a high critical current densitythrough the junction can be prepared by providing a solder, which has alower peritectic temperature than REBa₂ Cu₃ O_(x) -based superconductingmaterials to be joined and, upon cooling, becomes an oxidesuperconductor of the same type as the REBa₂ Cu₃ O_(x) -basedsuperconducting materials to be joined, aligning the materials to bejoined so as to have an identical orientation, partially melting thejunction and conducting orientation and growth in the junction.

EXAMPLE 1

Joining of a YBa₂ Cu₃ O_(x) -based material prepared by the melt processwas attempted. In this material, although a Y₂ BaCuO₅ phase having anaverage size of not more than 2 μm was dispersed therein, the matrixcomprised an oriented YBa₂ Cu₃ O_(x) superconducting phase free from alarge angle grain boundary. Plate samples were taken off from thismaterial and placed as shown in FIG. 5. In the drawing, the YBa₂ Cu₃O_(x) materials 3 to be joined, indicated by the numerals 4 and 5, had asize of 1×4×10 mm, and the YBa₂ CU₃ O_(x) material for use in joininghad a size of 1×2×8 mm. The joining area was 2 mm×2 mm. Regardingcrystal orientation, the direction vertical to the joining area was thec-axis, and also in the a-/b-axis, the materials were regulated so as tohave an identical orientation. The crystal orientation was judged fromcleavage plane, cracks and twin patterns. It is known that the cleavageand the crack occur within the ab plane while a twin boundary occurs ina place which is parallel to the c-axis and has an angle of 45° to thea-/b-axis.

In these materials, as shown in the cross-sectional view of FIG. 6, thejoining interface was covered with a powdery joining solder 6 in athickness of 0.5 mm. The junction solder was prepared by weighing a Yb₂O₃ powder, a BaCO₃ powder and a CuO powder in an element ratio of Yb toBa to Cu of 1.2:1.8:2.6, heating the mixture in a platinum crucible at1450° C. for 5 min, rapidly cooling the heated mixture on a copperhearth and pulverizing the cooled mixture.

The above assembly sample was heated to 985° C. on an aluminum plateover a period of 2 hr, gradually cooled from this temperature to 880° C.at a rate of 1° C./hr and then subjected to furnace cooling to roomtemperature. In the heat-treated sample, joining was confirmed at thejoining interface. Further, a silver film was formed on the sample inits portions 7 to 12 shown in FIG. 7 by using a sputtering device, andthe sample was heated in an oxygen gas stream at 600° C., graduallycooled from this temperature to 300° C. at a rate of 10° C./hr and thensubjected to furnace cooling to room temperature. In this case, theformation of a silver film was carried out for the purpose of flowing acurrent to measure the electric resistance, while the heat treatment inoxygen was carried out for the purpose of lowering the contactresistance during the measurement and bringing the sample to asuperconductor.

In this sample, a current was flowed across an electrode 7 and anelectrode 12, and the voltage was measured across electrodes 8 and 9 andacross electrodes 10 and 11. The measurement was carried out at liquidnitrogen temperature. As a result, it was found that no voltage (notmore than 0.5 μV) occurred across the electrodes 8 and 9 and across theelectrodes 10 and 11 even when a current of 200 A was flowed. Therefore,the joining has a critical current density of not less than 5000 A/cm²at 77K.

Further, the cross section of the joining between the electrodes 8 and 9was observed under a polarizing microscope. As a result, it was foundthat, as shown in FIG. 8, the joining could be successfully attained tosuch an extent that the joined portions were indistinguishable. In thepolarizing microscope, a grain boundary having a different crystalorientation should be distinguishable as a difference in contrast. Inthe photograph of FIG. 8, black streaks parallel to the longitudinaldirection of the sample and black spots are respectively cracks(occurring parallel to the ab plane) and voids.

FIG. 9 is an absorption electron image in the vicinity of the joininginterface. In the photograph, deep-color portions contain Yb having alarge atomic weight and constitute a joining layer. As can be seen fromthe drawing, an about 100-μm joining layer was formed in the joinedproduct. FIG. 10 shows a Yb distribution taken along a straight linecrossing the joining interface at a right angle. Such an analysis byEPMA was carried out for Y, Ba and Cu. As a result, it was found thatthe joining layer was composed mainly of a YbBa₂ Cu₃ O_(x) phase inwhich Y was slightly dissolved in a solid solution form. The aboveresults demonstrate that the two YbBa₂ Cu₃ O_(x) superconducting phaseshaving an identical orientation was joined with each other through aYbBa₂ Cu₃ O_(x) joining phase having the same orientation as the YbBa₂Cu₃ O_(x) superconducting phases.

EXAMPLE 2

Joining of a SmBa₂ Cu₃ O_(x) -based superconducting material prepared bythe melt process was attempted. In this material, although a Sm₂ BaCuO₅phase was dispersed therein, the matrix comprised an oriented SmBa₂ Cu₃O_(x) superconducting phase free from a large angle grain boundary.Samples were taken off from this material and placed in the same manneras in Example 1. As shown in the cross-sectional view of FIG. 6, thejoining interface of these materials was covered with a powdery joiningsolder 6 in a thickness of 0.5 mm. The joining solder was prepared byweighing and mixing together a Y₂ O₃ powder, a BaO₂ powder and a Cuopowder in an element ratio of Y to Ba to Cu of 1:2:3 and heat-treatingthe mixture in an oxygen gas stream at 800° C. for 10 hr.

The above assembly sample was heated to 1045° C. on an aluminum plateover a period of 2 hr, gradually cooled from this temperature to 950° C.at a rate of 1° C./hr and then subjected to furnace cooling to roomtemperature. In the heat-treated sample, it was confirmed that thematerials were successfully joined at the joining interface. Thereafter,the formation of a silver film and heat treatment in oxygen were carriedout in the same manner as in Example 1. In this sample, a current wasflowed across an electrode 7 and an electrode 12, and the voltage wasmeasured across electrodes 8 and 9 and across electrodes 10 and 11. Themeasurement was carried out at liquid nitrogen temperature. As a result,it was found that no voltage (not more than 0.5 μV) occurred across theelectrodes 8 and 9 and across the electrodes 10 and 11 even when acurrent of 200 A was flowed. Therefore, the junction has a criticalcurrent density of not less than 5000 A/cm² at 77K.

EXAMPLE 3

Joining of a YBa₂ Cu₃ O_(x) -based superconducting material prepared bythe melt process was attempted. In this material, although a Y₂ BaCuO₅phase having an average size of not more than 2 μm was homogeneouslydispersed therein, the matrix comprised an oriented YBa₂ Cu₃ O_(x)superconducting phase free from a large angle grain boundary. Sampleswere taken off from this material and placed in the same manner as inExample 1. As shown in the cross-sectional view of FIG. 6, the joininginterface of these materials was covered with a powdery joining solder 6in a thickness of 0.5 mm. The joining solder was prepared by weighing aYb₂ O₃ powder, a BaCO₃ powder and a CuO powder in an element ratio of Ybto Ba to Cu of 1.6:2.3:3.3, heating the mixture in a platinum crucibleat 1450° C. for 5 min, rapidly cooling the heated mixture on a copperhearth, pulverizing the cooled mixture and adding 10% by weight of Ag₂ Oto the powder.

For the above assembly sample, heat treatment and the formation of asilver film were carried out in the same manner as in Example 1. In thissample, a current was flowed across an electrode 7 and an electrode 12,and the voltage was measured across electrodes 8 and 9 and acrosselectrodes 10 and 11. The measurement was carried out at liquid nitrogentemperature. As a result, it was found that no voltage (not more than0.5 μV) occurred across the electrodes 8 and 9 and across the electrodes10 and 11 even when a current of 200 A was flowed. Therefore, thejunction has a critical current density of not less than 5000 A/cm² at77K.

EXAMPLE 4

Joining of a YBa₂ Cu₃ O_(x) -based superconducting material prepared bythe melt process was carried out, and properties of the junction in amagnetic field were evaluated. In this material, although a Y₂ BaCuO₅phase having an average size of not more than 2 μm was dispersedtherein, the matrix comprised an oriented YBa₂ Cu₃ O_(x) superconductingphase free from a large angle grain boundary. These materials and ajoining solder were placed as shown in a perspective view of FIG. 11.The three YBa₂ Cu₃ O_(x) -based superconducting materials 13 to bejoined had a length (in the direction of current flow) of 8 mm, a widthof 4 mm and a thickness of 2 mm. The joining solder 14 had a thicknessof about 1 mm, and the junction had an area of 4 mm×4 mm. Theorientation was regulated in such a manner that the direction ofthickness was that of c-axis with the directions of length and widthbeing those of a-/b-axis. In the drawing, an arrow 15 designates thedirection of the c-axis, and the lattice pattern represents thea-/b-axis.

The joining solder was prepared by weighing a Yb₂ O₃ powder, a BaO₂powder and a CuO powder in an element ratio of Yb to Ba to Cu of1.3:1.7:2.4, further adding 0.5% by weight of metallic platinum, millingthem together, calcining the mixture in an oxygen gas stream at 800° C.for 10 hr, pulverizing the calcination product and molding the powder,firing the molded product at 890° C. for 24 hr and taking off thejoining solder from the fired product.

The above sample assembly was heated to 975° C. over a period of 4 houron an aluminum plate while pressing the sample assembly by putting aweight of 150 g on the top of the sample located at the center of theassembly for the purpose of increasing the adhesion of the joining. Theassembly was held at that temperature for 4 hr, cooled to 920° C. over aperiod of one hr, gradually cooled from 920° C. to 840° C. at a rate of5° C./hr and then subjected to furnace cooling to room temperature. Forcomparison, an additional sample which had been subjected to furnacecooling from 920° C. to room temperature was prepared. For thiscomparative sample, the average rate of cooling from 920° C. to 840° C.was about 300° C./hr. The sample which had been gradually cooled at arate of 5° C./hr was designated as sample A, and the sample which hadbeen cooled at a rate of 300° C./hr was designated as sample B.

In both the samples A and B, it was found that the materials could befirmly joined at their joining interface. Further, for these samples,the formation of a silver film and the heat treatment in an oxygen gasstream were carried out in the same manner as in Example 1. The criticalcurrent across both ends with a junction sandwiched therebetween wasmeasured in a magnetic field. The magnetic field makes a right anglewith the direction of the c-axis. The results are shown in FIG. 12. Inthe graph, ◯ represents data for sample A with  representing data forsample B.

As shown in the drawing, the critical current of sample B was low andrapidly lowered in a magnetic field, whereas sample A had a criticalcurrent of 150 A in a magnetic field of 10 T. The critical currentcharacteristics in a magnetic field of not more than 8 T could not bemeasured due to the capacity of a sample holder.

The structure of these samples in their junctions was observed under apolarizing microscope. FIGS. 13 and 14 are photographs of planesperpendicular to joining areas respectively for samples A and B, and ineach photograph, a joining layer was present in the vertical directionin the photograph at its center. As is apparent from these photographs,no crystal grain boundary was observed in the junction of sample A,whereas the formation of polycrystals was observed in the joininginterface of sample B which had been rapidly cooled. The reason why thecritical current density of sample B was low is that polycrystals wereformed at the junction and the grain boundary served as a weak bond.

The joining area of sample A was shaved parallel to the ab plane of YBa₂CU₃ O_(x) by mechanical polishing to expose the surface, and the joiningarea was subjected to ω scanning using a quardriaxial goniometer toobtain an X-ray diffraction peak. The size of X-ray spot was about 4mm². FIG. 15 is a locking curve from the (006) plane, and it was foundthat the half-value width of the peak was about 1°. The ω scanning is atechnique commonly used for determining the orientation of highlyorientated grains, and the locking curve obtained by the ω scanningrepresents the grain orientaion. In this case, the abscissa representsthe breadth of angle at which the c-axis is distributed in the region of4 mm². In the oriented REBa₂ Cu₃ O_(x) bulk material used in the presentexample, the half-value width is in the range of from 0.50 to 10. Sincethe half-value width for the joining phase is also about 1°, it wasfound that the c-axis of YbBa₂ Cu₃ O_(x) constituting joining layer issubstantially perpendicular to the joining face and the orientationthereof is identical to that of the joined YBa₂ Cu₃ O_(x) bulk. Further,the observation of the twin pattern in the joining face under apolarizing microscope revealed that the twin pattern had the sameorientation as the YBa₂ Cu₃ O_(x) around the twin pattern. Therefore, itwas found that also in the a-/b-axis, the joining layer had the sameorientation as the YBa₂ Cu₃ O_(x) around the joining layer.

EXAMPLE 5

Then, the critical current density of several junctions having a crystalmisorientation was determined. The positional relationship between theYBa₂ Cu₃ O_(x) sample used and the joining solder was the same as thatin Example 4. The sizes of the sample and the joining solder were thesame as those in Example 4, except that the thickness of the joiningsolder was 0.3 mm.

At the outset, in order to provide a crystal misorientation in thedirection of the c-axis, the YBa₂ Cu₃ O_(x) to be joined and disposed atthe center of the assembly was placed so that the c-axis was slanted.This material was placed as shown in FIG. 16. In FIG. 16, the crystalorientation of YBa₂ CU₃ O_(x) materials 16, 18 is the same as that inExample 4. Specifically, the direction of an arrow 20 is the c-axis, andthe direction of the width and length of the sample is the a-/b-axis.The a/b-axis of YBa₂ Cu₃ O_(x) material 17 shown in FIG. 16 wasregulated so as to conform to those of the samples 16, 18 with respectto the rotation of the c-axis. On the other hand, the c-axis wasslanted, and this angle is expressed as z. In FIG. 16, the hatched planerepresents a plane parallel to the ab plane of the sample 17, and thec-axis is the direction of an arrow 21. The formation of a silver filmwas carried out in the same manner as in Example 1, and a current wasflowed across the samples 16 and 18 to measure the critical currentdensity of the joining area. The relationship between z and the criticalcurrent density at 77K and 5 T is summarized in the following table. Themagnetic field was perpendicular to the c-axis.

    ______________________________________                                                          Critical current                                            z (°)      density (A/cm.sup.2)                                        ______________________________________                                         0                Not less than 1250                                          10                185                                                         15                5                                                           20                2                                                           30                1                                                           ______________________________________                                    

From this table, it is apparent that the critical current density lowerswith an increase in crystal misorientation of the c-axis.

Then, in order to provide a crystal misorientation in the direction ofthe a-/b-axis, the YBa₂ Cu₃ O_(x) to be joined and located at the centerof the assembly was placed with it being rotated about the c-axis asshown in FIG. 17. In FIG. 17, the direction of the lattice representsthe direction of the a-/b-axis. Further, in FIG. 17, the YBa₂ Cu₃ O_(x)samples 22 and 24 had the same crystal orientation as in Example 4, andthe direction of an arrow is the c-axis of the three samples.Specifically, the direction of c-axis of the YBa₂ Cu₃ O_(x) 23 in FIG.17 is in agreement with that of the samples 22 and 24. However, thea-/b-axis of the YBa₂ Cu₃ O_(x) 23 is different from that of the samples22 and 24, and the angle which the a-/b-axis of the YBa₂ Cu₃ O_(x) 23makes with that of the samples 22 and 24 is expressed as y. In thedrawing, the hatched plane is a plane parallel to a plane surrounded bythe c-axis and the a-/b-axis of the sample 23. The assembly wassubjected to a treatment for the formation of a silver film in the samemanner as in Example 1, and a current was flowed across the samples 22and 24 to measure the critical current density of the junction area. Therelationship between y and the critical current density at 77K and 5 Tis summarized in the following table. The magnetic field wasperpendicular to the c-axis.

    ______________________________________                                                          Critical current                                            y (°)      density (A/cm.sup.2)                                        ______________________________________                                         0                Not less than 1250                                          10                85                                                          15                6                                                           20                3                                                           45                1                                                           ______________________________________                                    

From this table, it is apparent that the critical current density lowerswith an increase in crystal misorientation of the a-/b-axis.

As is apparent from the above experimental results, no high criticalcurrent density can be obtained when the difference in crystal axis,i.e., c-axis and a-/b-axis, between the materials to be joined is notless than 15°. Although the necessary critical current density variesdepending upon the applications, when the critical current density ofthe joined product with the crystal misorientation of the axis being notless than 15° is equal to that of polycrystalline materials, so that thefeatures of the present invention is substantially lost.

EXAMPLE 6

An experiment on the form of joining shown in FIG. 2 was carried out. Ina material to be joined, although a Y₂ BaCuO₅ phase having an averagesize of not more than 2 μm was dispersed therein, the matrix comprisedan oriented YBa₂ Cu₃ O_(x) superconducting phase free from a large anglegrain boundary. The materials and a joining solder were placed as shownin FIG. 18. The two YBa₂ Cu₃ O_(x) -based superconducting materials 27to be joined were provided by cutting disks having a diameter of 30 mmand a thickness of 7 mm from the center portion of disks. Regarding thecrystal orientation of the samples, the direction of the thickness ofthe disk was the c-axis. The joining solder 28 comprised the samematerial as that described in Example 4 and placed to have a size of31×7.5×0.5 mm. The resultant assembly was placed within a furnace sothat the joining plane was horizontal. Then, it was subjected to heattreatment for joining while applying a load of 400 g from the top of theassembly in the same manner as in Example 4, except that the rate ofgradual cooling from 920° C. to 840° C. was 2° C./hr. Further, forcomparison in the same manner as in Example 4, a sample which had beencooled from 920° C. to 840° C. at a rate of 300° C./hr was alsoprepared. The sample which had been cooled at a rate of 2° C./hr wasdesignated as sample C, and the sample which had been cooled at a rateof 300° C./hr as sample D.

In the two samples 27 which had been heat-treated as described above,strong joining was observed at the joining interface 31. The twosamples, however, were different from each other in texture at thejoining area. Specifically, no grain boundary was observed in thejoining interface of sample C, whereas the joining interface of sample Dwas in a polycrystalline form.

Then, these two samples were subjected to an oxygen enrichment treatmentat 450° C. for 100 hr in an oxygen gas stream and cooled in a magneticfield. Then, the trapped magnetic flux density was measured. At theoutset, a magnetic field of 1.3 T was applied in the direction ofthickness of the disk at room temperature, and the superconductor wascooled with liquid nitrogen with the magnetic field being held to removethe magnetic field. The two superconductors were in a magnetized state.A Hall probe 30 was adhered to the surface of the disk 27 at itspositions shown in FIGS. 19 and 20, and the magnetic flux density in adirection perpendicular to the surface of the sample was measured.

The results are shown in FIG. 21. In the graph, ◯ represents data forsample C, while  represents data for sample D. The magnetic fluxdensity distribution for sample C was in a large angular form. On theother hand, in sample C which had been cooled at a high rate and has apolycrystalline joining interface, the magnetic flux density was 0 inthe joining interface. This suggests that as shown in FIG. 19, in theinside of sample C, a large superconducting permanent current 29 flowsaround/circulate through the whole sample across the joining area,whereas in sample D having a polycrystalline joining interface 31, sincea large superconducting current cannot be flowed across the junction,the superconducting current 29 is detoured as shown in FIG. 20. Thesuperconducting permanent current flowing across the junction of sampleC estimated from the magnetic flux density distribution was about 8000A/cm².

Further, the experimental results suggest that when sample C is cooledin a non-magnetic field, shielding up to about 0.5 T is possible at thecenter of sample C.

EXAMPLE 7

An experiment on joining as shown in FIG. 3 was carried out. A materialto be joined comprised an oriented (Y₀.0 Gd₀.1)Ba₂ Cu₃ O_(x)superconductor free from a large angle grain boundary prepared by themelt process. This material and a joining solder were placed as shown inFIG. 3. The two (Y₀.9 Gd₀.1)Ba₂ Cu₃ O_(x) superconducting materials tobe joined had a length (direction of current flow) of 8 mm, a width of 2mm and a thickness (vertical direction in FIG. 1) of 2 mm, and thejoining plane had an area of 2 mm×2 mm. Regarding the orientation, thedirection of the thickness was the c-axis. The joining solder had alength of 6 mm, a width of 2 mm and a thickness of 2 mm and was preparedusing the same raw material and heat treatment method as in Example 4,except that the element ratio of Yb to Ba to Cu was 1.2:1.8:2.6.

The assembly comprising the samples thus disposed were heated to 995° C.over a period of 4 hr on an aluminum plate, held at that temperature for4 hr, cooled to 920° C. over a period of 1 hr, gradually cooled from920° C. to 850° C. at a rate of 1° C./hr and then subjected to furnacecooling to room temperature.

In the heat-treated assembly, joining was successfully attained in thejoining interface. The assembly was further subjected to a treatment forthe formation of a silver film and heat-treated in oxygen in the samemanner as in Example 1, and the critical current across both ends withtwo junctions being sandwiched therebetween was measured in a magneticfield. As a result, it was found that the critical current density in amagnetic field of 1.6 T was 5000 A/cm².

INDUSTRIAL APPLICABILITY

The joining method according to the present invention is considereduseful particularly for the joining of a bulk superconducting materialprepared by the melt process such as QMG process. As described above,this material is a monocrystalline (free from a large angle grainboundary) material prepared by taking advantage of crystal growth. Inthis process, it is difficult to prepare a large material having a sizeexceeding a certain value, and working and joining which form a grainboundary detrimental to a high critical current density cannot becarried out, which limits the field of applications. On the other hand,the joining prepared according to the method of the present inventionfree from a brain boundary causative of a lowering in critical currentcan be widely applied to current leads, magnetic shields in a magneticfield having a medium strength, bulk magnets and superconducting coils,for example, as shown in FIGS. 22 to 25.

FIG. 22 shows a current lead formed by successively joining a number ofrod-shaped superconductors 32 to one another using a solder 33 to form along rod. In this current lead, a current flows as indicted by an arrow34.

FIGS. 23 and 24 show a magnetic field or a bulk magnet formed by joininga plate superconductor 32 using a solder 33 to form a wider plate. Inthis case, a current flow as indicated by an arrow 34. In FIG. 23,joining is carried out by forming a solder layer 33 between the platesuperconductors 32, and in FIG. 24, the plate superconductors 32 arejoined together by using a joining material 35.

FIG. 25 shows a superconducting coil. A superconducting coil can beproduced by forming a spiral superconducting path 38 using an insulatingmaterial 36 and a cut (capable of being filled with an insulatingmaterial) 37. Since, however, there is a limitation on the size (coillength), end portions of superconducting paths 38 of superconductingcoils are joined together using a solder 39 to successively conductjoining to a desired coil length. Numeral 40 designates an external leadconnected to a superconducting coil.

We claim:
 1. A joined product comprising a plurality of RE¹ Ba₂ Cu₃O_(x) -based monocrystalline superconducting material segments, witheach segment having a joining area and an identical crystal orientation,said segments joined with each other at said joining areas with an RE²Ba₂ Cu₃ O_(x) -based superconducting phase monocrystalline solder formedby a melt process, whereinRE¹ is at least one element selected from agroup consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, RE² isat least one element selected from a group consisting of Y, Nd, Sm, Eu,Gd, Dy, Ho, Er, Tm, Yb and Lu, said RE² Ba₂ Cu₃ O_(x) -basedsuperconducting phase monocrystalline solder has a lower peritectictemperature than said RE¹ Ba₂ Cu₃ O_(x) -based monocrystallinesuperconducting material segments, and said RE² Ba₂ Cu₃ O_(x) -basedsuperconducting phase monocrystalline solder has the same crystalorientation as said RE¹ Ba₂ Cu₃ O_(x) -based monocrystallinesuperconducting material segments.
 2. A joined product according toclaim 1 wherein said RE² Ba₂ Cu₃ O_(x) -based superconducting phasemonocrystalline solder is sandwiched between said RE¹ Ba₂ Cu₃ O_(x)-based monocrystalline superconducting material segments.
 3. A joinedproduct comprising a plurality of RE¹ Ba₂ Cu₃ O_(x) -basedmonocrystalline superconducting material segments, with each segmenthaving a joining area and an identical crystal orientation, saidsegments joined with each other at said joining areas with an RE² Ba₂Cu₃ O_(x) -based superconducting phase monocrystalline solder formed bya melt process, whereinRE¹ is at least one element selected from a groupconsisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, RE² isidentical to RE¹, said RE² Ba₂ Cu₃ O_(x) -based superconducting phasemonocrystalline solder has added thereto at least one element selectedfrom a group consisting of Ag and Au whereby said RE² Ba₂ Cu₃ O_(x)-based superconducting phase monocrystalline solder has a lowerperitectic temperature than said RE¹ Ba₂ Cu₃ O_(x) -basedmonocrystalline superconducting material segments, and said RE² Ba₂ Cu₃O_(x) -based superconducting phase monocrystalline solder has the samecrystal orientation as said RE¹ Ba₂ Cu₃ O_(x) -based monocrystallinesuperconducting material segments.
 4. A joining method formonocrystalline superconducting material segments comprising:providingat least two RE¹ Ba₂ Cu₃ O_(x) -based monocrystalline superconductingmaterial segments, wherein RE¹ is at least one element selected from agroup consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu,aligning at least two of said RE¹ Ba₂ Cu₃ O_(x) -based monocrystallinesuperconducting material segments to be joined so that they have anidentical crystal orientation, inserting a solder between faces of saidoriented monocrystalline RE¹ Ba₂ Cu₃ O_(x) -based superconductingmaterial segments to be joined to form an assembly, wherein said solderis at least one member selected from a group consisting of an RE² Ba₂Cu₃ O_(x) -based superconducting material and a raw material for an RE²Ba₂ Cu₃ O_(x) -based superconducting material, wherein RE² is at leastone element selected from a group consisting of Y, Nd, Sm, Eu, Gd, Dy,Ho, Er, Tm, Yb and Lu, wherein said RE² Ba₂ Cu₃ O_(x) -basedsuperconducting material has a lower peritectic temperature than saidmonocrystalline RE¹ Ba₂ Cu₃ O_(x) -based superconducting materialsegments to be joined, heating said assembly to a temperature above theperitectic temperature of said solder and below the peritectictemperature of said monocrystalline RE¹ Ba₂ Cu₃ O_(x) -basedsuperconducting material segments to be joined, said temperature ofheating said assembly decomposing said member selected from a groupconsisting of said RE² Ba₂ Cu₃ O₃ -based superconducting material andsaid raw material for said RE² Ba₂ Cu₃ O_(x) -based superconductingmaterial into a Ba, Cu, and O liquid phase and an RE₂ ² BaCuO₅ solidphase, gradually cooling said heated assembly, said gradual cooling ofsaid heated assembly providing an RE² Ba₂ Cu₃ O_(x) -basedsuperconducting phase monocrystalline solder joining said RE¹ Ba₂ Cu₃O_(x) -based monocrystalline superconducting material segments, withsaid RE² Ba₂ Cu₃ O_(x) -based superconducting phase monocrystallinesolder having the same crystalline orientation as said RE¹ Ba₂ Cu₃ O_(x)-based monocrystalline superconducting material segments joined by saidsolder.
 5. A joining method for monocrystalline superconducting materialsegments comprising:providing at least two RE¹ Ba₂ Cu₃ O_(x) -basedmonocrystalline superconducting material segments, wherein RE¹ is atleast one element selected from a group consisting of Y, Nd, Sm, Eu, Gd,Dy, Ho, Er, Tm, Yb and Lu, aligning at least two of said RE¹ Ba₂ Cu₃O_(x) -based monocrystalline superconducting material segments to bejoined so that they have an identical crystal orientation, placing asolder on said monocrystalline RE¹ Ba₂ Cu₃ O_(x) -based superconductingmaterial segments to be joined to form an assembly, wherein said solderis at least one member selected from a group consisting of an RE² Ba₂Cu₃ O_(x) -based superconducting material and a raw material for an RE²Ba₂ Cu₃ O_(x) -based superconducting material, wherein RE² is at leastone element selected from a group consisting of Y, Nd, Sm, Eu, Gd, Dy,Ho, Er, Tm, Yb and Lu, wherein said RE² Ba₂ Cu₃ O_(x) -basedsuperconducting material has a lower peritectic temperature than saidmonocrystalline RE¹ Ba₂ Cu₃ O_(x) -based superconducting materialsegments to be joined, heating said assembly to a temperature above theperitectic temperature of said solder and below the peritectictemperature of said monocrystalline RE¹ Ba₂ Cu₃ O_(x) -basedsuperconducting material segments to be joined, said temperature ofheating said assembly decomposing said member selected from a groupconsisting of said RE² Ba₂ Cu₃ O_(x) -based superconducting material andsaid raw material for said RE² Ba₂ Cu₃ O_(x) -based superconductingmaterial into a Ba, Cu, and O liquid phase and an RE² ₂ BaCuO₅ solidphase, gradually cooling said heated assembly, said gradual cooling ofsaid heated assembly providing an RE₂ Ba₂ Cu₃ O_(x) -basedsuperconducting phase monocrystalline solder joining said RE¹ Ba₂ Cu₃O_(x) -based monocrystalline superconducting material segments, withsaid RE² Ba₂ Cu₃ O_(x) -based superconducting phase monocrystallinesolder having the same crystalline orientation as said RE¹ Ba₂ Cu₃ O_(x)-based monocrystalline superconducting material segments joined by saidsolder.